A belt transmission device is used to transmit the rotation of an engine crankshaft to various vehicle engine accessories. Such a belt transmission device is shown in FIG. 13, which includes a pulley arm 53 provided at a slack side of a belt 51 so as to be pivotable about a pivot shaft 52. The pulley arm 53 rotatably carries a tension pulley 54 at its end remote from the pivot shaft 52. The above-mentioned hydraulic auto-tensioner A is arranged to apply an adjusting force to the pulley arm 53 to bias the pulley arm 53 in the direction in which the tension pulley 54 is pressed against the belt 51, thereby keeping constant the tension of the belt 51.
One hydraulic auto-tensioner A used in such belt transmission devices includes a sleeve, and a rod slidably inserted in the sleeve to define a pressure chamber in the sleeve. The hydraulic auto-tensioner is configured such that when a pushing force is applied from the belt 51 to the rod, hydraulic oil in the pressure chamber flows through a minute gap defined between the radially inner surface of the sleeve and the radially outer surface of the rod, thereby generating a hydraulic damping force in the pressure chamber, which dampens the pushing force.
The hydraulic damping force generated in this hydraulic auto-tensioner is substantially proportional to the pushing force. That is, the hydraulic damping force increases as the pushing force increases.
This auto-tensioner is therefore incapable of preventing over-tensioning of the belt, and could reduce the durability of the belt.
JP Patent Publication 2011-27238A proposes a hydraulic auto-tensioner which is free of this problem. In this auto-tensioner, a relief valve is mounted in a valve fitting hole formed in the lower end portion of the rod, with the pressure chamber communicating with the reservoir chamber through the valve fitting hole and a communication passage formed in the rod. With this arrangement, if the pressure in the pressure chamber exceeds the set pressure of the relief valve, the relief valve is adapted to open, releasing hydraulic oil in the pressure chamber into the reservoir chamber through the communication passage, thereby preventing over-tensioning of the belt.
In the hydraulic auto-tensioner disclosed in JP Patent Publication 2011-27238A, when the relief valve opens, hydraulic oil in the pressure chamber is instantly released into the reservoir chamber, so that the pressure in the pressure chamber drops sharply, which could cause separation of air dissolved in hydraulic oil. The separated air remains in the pressure chamber, and will be compressed when pushing force is applied to the hydraulic auto-tensioner, thus deteriorating the hydraulic damping function.
The relief valve of this hydraulic auto-tensioner includes a valve seat press-fitted in the valve fitting hole, a valve body for opening and closing a valve hole formed in the valve seat, a valve spring biasing the valve body toward the closed position, and a spring seat mounted between the valve spring and the valve body and having a shaft portion protruding from the top surface thereof. The valve fitting hole has a top wall which restricts the moving distance of the spring seat by abutting the shaft portion, thereby restricting the degree of opening of the relief valve. Thus, the top end surface of the shaft portion tends to become worn or may be deformed by repeatedly abutting the top wall of the valve fitting hole. The thus worn or deformed top end surface of the shaft portion may close the opening of the communication passage. For this reason, the relief valve of this auto-tensioner is not sufficiently reliable.
Since the shaft portion is small in diameter, it is difficult, and thus costly, to form the shaft portion.